Gemara

10b (Mishnah): The following Chata'os must die -- Vlad Chatas, Temuras Chatas and she'Mesu Ba'aleha (its owner died)...

11a (Mishnah): If a Nazir designated money for his Korban Chatas, it goes to Yam ha'Melach. One may not benefit from it, but Me'ilah does not apply;

Pesachim 50b: Bnei Beishan were stringent not to go from Tzur to Tzidon on Erev Shabbos (the market day, lest they not have time to prepare for Shabbos).

Question (the next generation): 'Must we must follow this stringency? Our (wealthy) ancestors could be stringent. We cannot!'

Answer (R. Yochanan): Since your ancestors accepted this, you must follow it - "Shma Beni Musar Avicha v'Al Titosh Toras Imecha."

(Beraisa): If some people (mistakenly) forbid certain things, you may not permit them to them.

(Rav Chisda): This refers to Kusim (lest they come to be lenient about ma90 that are truly forbidden).

Question: It applies to others also!

(Beraisa): Cases occurred in which Hillel, Yehudah and Shimon, sons of R. Gamliel, refrained from telling people in Kavul, Biri and Ako that they were stringent unlike the Halachah!

Answer: Since Chachamim are scarcely found overseas, people there are (unlearned) like Kusim.

Rishonim

Rosh (Pesachim 4:3): If some people (mistakenly) forbid certain things, you may not permit them to them. R. Nisim Ga'on says that this is only if they know that it is permitted, but they want to be stringent. If they err and think that it is forbidden, you may permit them. The Yerushalmi explains the case of Bnei Beishan like this. If one knows that it is forbidden, you may not permit him automatically, for it is like a vow. You can permit him through regret that he conducted like a vow.

Rivash (399): People of a city can make enactments, fences and agreements, and fine one who transgresses them, like it says in Bava Basra (8b). The enactments affect also minors. If not, they would have to renew the enactment every day for children who became Bar Mitzvah. Also those who will be born are included. After Pilegesh b'Givah, the other tribes swore not to marry their daughters to men of Binyamin. This Isur lasted (many generations) until they expounded "from us", but not from our children (Bava Basra 121a). If not for this, the Isur would apply to all generations. The Ramban (Mishpat ha'Cherem) says that every acceptance of the Rabim is on them and their children, like the acceptance of the Torah. The same applies to Megilah, fasts, and other matters. This is even for a custom without a formal acceptance, like in Beishan. If a city could not enact for future generations, all the more so their customs would not be binding on the next generation!

Poskim

Shulchan Aruch (YD 214:2): Acceptance of the Rabim takes effect on them and their descendants. This is even for matters that they did not explicitly agree to accept on themselves, but they conducted like this for a fence for Torah.

Beis Yosef (DH u'Mah she'Chasav Aval): A custom to be stringent based on a mistake is not like a vow. R. Nisim Ga'on and the Yerushalmi say so. If they knew that it is permitted and wanted to make a fence, you may not permit them until they request to permit it, like other vows. The Gemara says that one may not permit (even though regret) to Kusim or in a place where they are not Bnei Torah, or if their fathers knew that it is permitted and chose to be stringent, and the children kept the custom and think that letter of the law it is forbidden, like Bnei Beishan. One may not permit them automatically, rather, only through regret, like other Nedarim. Therefore, we are not concerned lest they extrapolate.

Pischei Teshuvah (4): Ra'anach (Mayim Amukim 12) wrote that if they conducted like a lenient opinion, and now want to forbid, they may. We are concerned for maligning previous generations only regarding marital matters. However, the Magen Avraham (OC 32:48) connotes unlike this.

Shulchan Aruch (228:27): If a Tzibur made an enactment or Cherem, they can permit it even within the time it was enacted for, even if the reason for the enactment still applies.

Shach (82): It is as if there is a stipulation about such Cheramim, that they can be permitted whenever they want.

Maharashdam (YD 40): If one was stringent about something that all agree that it is permitted, some say that he may permit his stringency through regret, like Heter Nedarim. However, if there was an argument, and in a place the custom is like the stringent opinion, this can never be permitted. The Rashba holds that a custom to be stringent about something permitted can never be permitted. It is more stringent than a vow. It is like an Isur Torah, which cannot be permitted. Even though the Rashba was not sure about this, the Tur brings an opinion that definitely says so. The Rosh and others argue, but we can say that this is only when the matter was totally permitted. The custom was like a new Isur, i.e. like a vow, which can be permitted. If there was an argument, and in a place the custom is like the stringent opinion, this is not like a vow. The stringent opinion holds that it was always forbidden! I say so even though the Ran connotes unlike this. Also, perhaps one may ask to permit only a new stringency, but not something that their fathers were stringent about. The case of Bnei Beishan proves this. R. Yochanan did not permit them through a Pesach, i.e. now they are poor and they cannot be stringent. The Beis Yosef connotes unlike this. I do not know his source.

Pri Chodosh (OC 496 DH Shemini): If people in a place had a custom to be stringent about a matter for a fence or Prishus (separation), Maharashdam says that it cannot be permitted if their fathers already did so. He learns from Bnei Beishan. Also Maharik 144 says so. If initially the custom was mistakenly established, i.e. they believed that it is forbidden, they, and all the more so their children, can permit this. It was a mistaken acceptance. However, if initially it was established for a fence and stringency, it cannot be permitted. Really, this does not support Maharashdam, for we hold that a custom for a fence can be permitted. Maharik holds that even the fathers cannot permit it. Why does he say 'all the more so their children can permit this'? If so, those who hold that the fathers can permit, all the more so the children can permit! Maharashdam says oppositely. One may not rely on the Maharik. The Rosh wrote oppositely. I.e. we do not permit a stringency automatically, but we permit through regret, like a Neder.

Pri Chodosh: Some great Chachamim hold like Maharik, but he did not mention them. He learned from the Gemara. How can the Rosh explain Bnei Beishan? The Rosh equates children to fathers, unlike Maharashdam. The Beis Yosef explains like this. This is primary. The Yerushalmi is like this. It brings the case of Bnei Beishan, and asked 'one cannot permit his vow?!' It (initially) answered 'one who vowed can permit his own vow. Here, the fathers vowed.' I.e., therefore the children cannot permit it. This is like Maharashdam's reasoning. The Yerushalmi countered 'all the more so they are permitted!' I.e. even the fathers, who vowed, could permit it, so all the more so the children, who did not vow. It is more lenient for them. It answered that Rebbi, who forbade them, was a Talmid of R. Yehudah, who forbids according to letter of the law. Therefore the custom cannot be permitted. We learn three things from this. 1) A fence can be permitted. The Yerushalmi asked incredulously 'one cannot permit his vow?!' 2) Just like the fathers can be permitted, also, in fact all the more so, the children can be. 3) The Yerushalmi said that Rebbi was a Talmid of R. Yehudah. This connotes that if not, i.e. if he did not hold like R. Yehudah who forbids, he would have permitted them.

Pri Chodosh: We concerned dispel the proof from Bnei Beishan. Perhaps they merely asked whether letter of the law it is permitted. Rebbi answered that it is forbidden due to Shma Beni. Or, we can say like the Ran, that if the fathers made a fence to forbid something permitted, and their children, who are not Bnei Torah, think that letter of the law it is forbidden, we do not reveal that it is only a custom, lest they be lenient. Likewise, we permit only if they seek to permit through regret. This is why Rebbi did not permit by himself.

Turei Even (Rosh Hashanah 28b DH v'Lo): If a man died and his son offers his Korban, he cannot uproot the Hekdesh through regret.

Zayis Ra'anan (2:22): It seems that children cannot permit their fathers' Hekdesh through regret. If not, why must a Chatas die if its owner died? It is not Batel even if it became mixed with 1000! Even if one separated money for his Chatas, it must perish. This is even if it is still in his Reshus and he did not yet give them to the Gizbar (Tosfos Nazir 24a DH Demei Chatas). We do not say that the heirs can uproot the Hekdesh through regret! The owner himself can uproot his Hekdesh until Shechitah and Zerikah.

Zayis Ra'anan: Really, this is not a proof. If so, why must the other Chata'os ha'Mesos die even though the owner is alive? The Ran explains that a Pesach and regret make the vow like a mistake. It was not a total mistake, therefore he must ask a Chacham. If it was a total mistake, it is Batel automatically, and does not need a Heter. In Bava Kama (110b) the Gemara suggested that if the owner of a Chatas died, it should revert to Chulin, for he made it Hekdesh only in order to offer it. It answered that a tradition from Moshe from Sinai teaches the animal is left to die. The tradition overrides a total mistake, so all the more so one cannot permit the Hekdesh through Pesach and regret which makes it like a mistake.

Zayis Ra'anan: Perhaps we can bring a proof from the Rashba (Chulin 18b DH v'Rebbi), who holds that a custom based on an error is like a fence according to other Poskim. Rebbi permitted Beis She'an (regarding Ma'aser) based on testimony that R. Meir was lenient there, even though they were stringent for generations (Chulin 6b). We must say that this was through Heter (like for vows). Just like he holds that children can permit (like vows) a custom based on error, other Poskim say the same about a fence. Do not say that nowadays is different, for we are not Bnei Torah. Surely some Kusim were in Beis She'a. Also, Tosfos (Pesachim 51a DH Iy) asked from Beis She'an, and did not say that they were (Bnei Torah, and hence) different. However, this is not a good proof. Letter of the law, an error is permitted automatically, but a fence requires Pesach and regret.